Removing foreign material from the surface of workpieces is a critical element in the manufacturing of semiconductor devices. The presence of contaminants or foreign material can lead to critical failures or prematurely shortened life spans of these devices.
In manufacturing semiconductor devices, silicon workpieces known as "wafers" are used. Wafers typically have a flat, circular disk-like shape. Wafers are initially sliced from a silicon ingot and, thereafter, undergo multiple masking, etching, and dielectric and conductor deposition processes to create microelectronic structures and circuitry on the wafers. The surface of a wafer undergoing these processes typically must be polished or planarized between processing steps to ensure proper flatness, thus permitting use of photo lithographic processes for building additional dielectric and metallization layers on the wafer surface.
Chemical Mechanical Planarization ("CMP") machines have been developed to polish or planarize silicon wafer surfaces to the flat condition necessary for manufacture of integrated circuit components and the like. CMP processes and machines are known in the art and are described in several U.S. Patents. Examples include U.S. Pat. No. 4,805,348, issued in February 1989, to Arai, et al.; U.S. Pat. No. 4,811,522, issued in March 1989 to Gill; U.S. Pat. No. 5,099,614, issued in March, 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued in July, 1994 to Karlsrud et al.; U.S. Pat. No. 5,476,890, issued in December 1995 to Masayoshi et al.; U.S. Pat. Nos. 5,498,196 and 5,498,199, both issued in March, 1996 to Karlsrud et al.; and U.S. Pat. No. 5,558,568, issued in September 1996 to Talieh et al.
CMP processing typically requires the introduction of a polishing slurry onto the surface of the wafer as it is mechanically polished on a polishing pad. The slurry typically contains fine abrasive particles such as silica and alumina dispersed in an alkali or acidic medium, and also may incorporate chemical cleaning compositions such as surfactants. Consequently, CMP processing of the wafer may leave contaminants such as leftover slurry particles, unwanted metallic ions (sodium, potassium, iron, chromium, nickel, manganese, zinc, titanium, etc), and micro scratch marks on the surface of the wafer. To remove these surface contaminants, the wafers are typically cleaned in a post-CMP cleaning machine.
In a conventional post-CMP cleaning machine, wafers are scrubbed by a set of scrub brushes using deionized ("DI") water, ammonia, or various other chemical solutions. The wafers also may be cleaned using a megasonic cleaning process whereby the workpieces are subjected to high frequency ultrasonic energy while being immersed in either DI water or ammonium hydroxide. Although scrub cleaning and megasonic cleaning may remove some surface contaminants, they may not remove the surface stress, implanted metallic ions, micro scratch marks, and various other unwanted contaminants or defects which may be contained within the layer of oxide damaged by CMP processing of the wafer.
Alternatively, a suitable cleaning solution such as hydrogen fluoride ("HF") may be used in combination with mechanical action to clean wafers. Although the cleaning solution will remove some of the damaged oxide layer from the wafers, the application of mechanical action in combination with an etching solution may result in introduction of sub microscopic scratches and the possibility for the addition of particles from the means used to apply the mechanical action. Additionally, safety and waste disposal concerns may be further disadvantages of this cleaning method.
Typically, CMP planarization and post-CMP cleaning are performed in independent machines. After the wafers are planarized on a CMP planarization machine, the wafers are transferred to a post-CMP cleaning machine. The time required to transfer the wafers increases the overall processing time and increases the likelihood that contaminants will dry on the surface of the wafers. To help prevent contaminants from drying on the surface of the wafers, the wafers are transported in a wet environment which requires special handling and equipment. Transporting workpieces from one machine to another has the additional disadvantage of increasing the risk of breaking the wafers.
After post-CMP cleaning, a chemical-etch cleaning process may be employed to remove the damaged layer of oxide from the wafers. In a conventional chemical-etch cleaning process, wafers are placed vertically into a static bath of chemical solution such as HF solution. This method, however, has several short-comings. For example, the HF solution gradually degrades as the cleaning operation is repeated, thus making it difficult to control the amount of oxide layer removed from the wafers. Additionally, the recirculated HF solution may incorporate additional particles on the surface of the wafers. Air bubbles adhering to the surface of the wafers as they are placed into the HF solution also may result in nonuniform etching of the wafers. Further, the reaction of silicon oxide with HF results in the formation of silicon fluoride and water molecules. Both the silicon fluoride and water molecules tend to stay at the interface between the wafer surface and the HF solution, which may result in low etch rate and nonuniform etching of the wafers. More particularly, the water remaining at the interface will dilute the concentration of the HF at the wafer surface, thus resulting in etch nonuniformity and lower etch rates.
Typically, chemical-etch cleaning of wafers is performed on special wet benches designed to handle hazardous chemicals. After the wafers are cleaned in a post-CMP cleaning machine, the wafers are transported to a chemical-etch cleaning machine. Transporting the wafers from one machine to another again increases the overall processing time, the likelihood that contaminants will dry on the surface of the wafers, and the risk of breakage. Therefore, a need exists for an apparatus to remove contaminants and damaged oxide layers from the surface of workpieces which overcomes the various short-comings associated with existing conventional methods.